Frequency response control



'Devn 11,1945.'

C. D. KNIGHT FREQUENCY RESPONSE CONTROL.

2 Sheets-Sheet 1 Filed Feb. 21. '1944 Dec. 11, 1945. c. D. KNIGHTFREQUENCY RESPONSE CONTROL Filed Feb. 2l, .1944 2 Sheets-Sheet 2 45F/IEG 49 vVARIABLE OPENING MENsIoNs 4.

,I'NVENTOR DANNY D T T Las C WI H AMP.

FR EQUENCY Patented Dec. ll, 1945 I,

FREQUENCY RESPONSE CONTROL Cosler Donald Knight, Chicago, Ill., assignorto Zenith Radio Corporation, a corporation of v Illinois ApplicationFebruary 21, 1944, Serial No. 523,379

17 Claims.

This invention relates to tone controls for electro-acoustic apparatusand particularly to tone controls in hearing aid apparatus. f

In present day hearing aid apparatus, there is supplied anelectro-acoustical transducer for converting sound energy Aintoelectrical energy, an amplifier for amplifying such electrical energy,

'and a second electro-acoustical transducer for converting the amplifiedelectrical energy into sound energy. The over-all frequency responsecharacteristic of such apparatus is determined to a large extent by thefrequency response characteristic of each one of the transducers andaccordingly such over-all frequency response characterlstic is alteredin a relatively small degree by providing a so-called tone r tuningcontrol in the amplifying circuit.

By the term over-al1 frequency response characteristic of acousticalapparatus it is understood that reference is made to the variation ofthe intensity of sound energy delivered at the output circuit of theapparatus as a function of fre,- quency over the range of frequencies inthe audible range when a constantJ intensity signal of correspondingfrequencies in such range is applied to the input circuit of theapparatus.

Also, by the term electro-acoustical transducer, it is understood thatreference is made to electro-acoustical apparatus having as its functionthe conversion of sound energy into electrical energy, as in amicrophone of the carbon, magnetic or piezoelectric type, or apparatushaving as its function the conversion of electrical energy into soundenergy as in a reproducer of the carbon, magnetic or piezoelectric type,such reproducer being, in hearing aid apparatus, either of the boneconduction or air conduction type.

The frequency response characteristics of such' transducers havingmechanical vibratory elements are determined largely by the mechanicalresonance, as distinguished from electrical resonance, properties of thevibratory elements whereby the over-all frequency responsecharacteristic of the composite hearing aid apparatus is determinedlargely by the mechanical properties of the vibratory elements in thetwo transducers.

When an attempt is made to match the over-al1 frequency responsecharacteristic of such hearing aid apparatus to the frequency responsech'aracteristic of ears of persons having different types and forms ofhearing deficiencies, the frequency at `which the individual vibratoryelement in the transducer is resonant,l mechanically speaking, assumesparamount importance, and which no means are present to vary thefrequency at which such vibratory elements are mechanically resonant.the matching operation is accomplished, but

usually unsatisfactorily, by electrical means such as by the use ofelectric filter means or by tuning electrically the vibratory elementswhich' might be piezoelectric or magnetic members.

It is accordingly an object of the present invention to provide inapparatus, such as hearing aid apparatus, a simple control preferablyoperable by the user, for variably controlling the mechanical resonantfrequency of vibratory elements in such apparatus whereby the over-allfrequency characteristic of such apparatus may be adjusted to supplyaccurately those deficiencies needed by the user forA satisfactoryhearing or sound reproduction. l

It is understood that the tone quality of transducershas been varied inthe past by enclosing a vibratory element within a closed space withvariable size openings in the enclosing member. in such cases, the airspace within which such vibratory elements move was defined bydimensions one or more of which was greater than onequarter of the wavelength of sound waves in the audible range. With this thought in mind,it is understood that the present invention has special reference totone controls for transducers having vibratory elements confined withina space whose linear dimensions are less than one-quarter of the Wavelength of sound in the audible range. y

It is realized that ordinarily change of size of known elementsaccompanied only by a change in degree and not of kind is not the basisof invention. A transducer constructed in accordance with' the teachingsof the present invention has as its function 'to change the mechanicalresonant frequency of the vibratory elementand not, as

in the prior art, to adapt a space surrounding a vibratory element foraccentuation of relatively unchanged characteristics in the resonantfrequency of such vibratory element.

Another object of the present invention is to -provide inelectro-acoustic apparatus an improved tone control characterized by itssimplicity, which' is essentially of a, mechanical nature.

Another object of the present invention is to provide an improvedelectro-acoustical transducer having a vibratory member whose mechanicalresonant f-requency may be varied to control adjusted for satisfactoryunderstanding of sounds by persons having many different types ofhearing deciency, such as a deciency in hearing low, medium and/or highfrequency audible sounds.

The features of the present invention which are believed to be novel areset forth with' particularlity in the appended claims. This inventionitself, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may best be understood byref-- erence to the following description taken in connection with theaccompanying drawings in which:

Figure 1 sh'ows generalized apparatus embodying the present invention;

Figs. 2-4 show means for controlling a characterlstic of the apparatusshown in Fig. l;

Figs. 5-8 show apparatus embodying the present invention, Fig. 8 showingthe apparatus ad- :lusted differently;

Fig. 9 shows modified apparatus embodying the present invention; Y

Fig. 10 shows frequency response characteristics of apparatus embodyingthe present invention.

The present invention -has applicability to radio, television oraudio-frequency translating systems incorporating a vibratory memberwhose mechanical resonant .frequency affects the fre.

.to the transducer I 2.

fied electrical variationsare further amplified by electron dischargedevice Il before being applied Transducer II, which produces electricalvariquency response characteristics of 'such systems tively: soundenergy into electrical energy. a twostage amplifier I Aiii foramplifying such electrical energy, and a second electro-acousticaltransducer I2 for converting the amplied electrical energy into soundwaves.

The particular elements of this device Ill, how-y The first transduceril and second transducer I2 may be of the carbon, magnetic orpiezoelectric type having a vibratory element. 'I'he amplifying circuitincluding device i9 connecting the two transducers is described and thepatentable l features thereof claimed in the copending patentapplication, Serial No. 504,958, filed October 4, 1943, of John G.Prentiss and assigned to the same assignee as this present application.

.The transducers Ii and i2 are characterized by the fact that themechanical resonant frequency of a vibratory element in each one of thetransducers may be variably controlled by means shown in Figs. 5-8 anddescribed later. By the term mechanical resonant frequency of avibratory element, it is understood that reference is made to one of aplurality of frequencies at v which a vibrato'ry member, due 'toparticular combinations ofcompliant and inertia forces acting thereon,vibrates at larger amplitudes at those frequencies than at otherfrequencies, even though the same 'energy but of corresponding differentfrequency is applied in the same manner to such vibratory member. Asiswell understood and known by those skilled in the electroacousticaltransducer art, such vibratory element/s may n'ot only exhibit arelatively large amplitude of vibration at which is termed a fundamentalfrequency, but also at harmonics or sub-harmonics of such fundamentalfrequency, or in certain cases, noticed particularly in piezoelectriccrystals, at different modes of vibration.

In 1, the hearing aid circuit incorporates between the main control gridi5 and thecathodel ations in response to sound waves impinging thereon,is effectively connected through conductor E2 and through rcouplingvmeans 33 between the main control grid I5 and the grounded filamentarycathode of discharge device I0. A grid leak resistance I6 is connectedbetween input grid I5 and ground to by-pass continuous current flowingaround transducer II between control grid I5 and the cathode ofdischarge device I0.

It is noted that substantially no grid current flows 'through resistanceI6 since grid l5 is at negative potential with respect to the cathode ofdevice Ill, such negative bias potential beingI provided by connectinggrid I5 through resistance I6 to the grounded negative terminal ofsource 40 having its terminals connected to opposite terminals of thecathode of device I0.

Electron discharge device l0 is of the pentagrid type in commercial useand may be, for example, of the type commonly known as the 1R5.

ever, are connected in a linear high gain amplifier circuit. Largegain'is realized when device il! is. connected in themanner hereinafter`described and such large gain is substantially independent of theamplitude of a signal applied of discharge device i9.

In generaL-device i9 is connected so as to be eectively two amplifiersin cascade with regeneration between the two amplifiers. In addition tothe main. control grid i5, discharge device ill has what is termed asecond control grid I8, a suppressor grid i9 connected to the cathode, amain anode 20, and a pair of screen electrodes 2l and 22 on oppositesides of the second con-- trol grid I8.

vativo terminal is grounded and whose positive terminal is connected tothe main anode 20 of discharge device I0 through a series circuitincluding adjustable voltage dropping and gain control resistance 24 andoutput' coupling resistance 25. Eelelctrodes 2| ,and 22 are connectedtogether and are maintained positive with respect to the cathode ofdevice IG by connection to the positive terminal of voltage source 23through the series circuit including voltage dropping and gain controlresistance 24 and coupling resistance 26. The continuous operatingpotential of the second control grid I8 is established by connecting itto ground and the cathode of discharge device Ill through resistance 2l;

When alternating current signals are impressed between fthe main controlgrid I5 and cathode of discharge device Ill, substantially all of thealternating output voltage appears across the output coupling resistance25, a by-pass capacitance 28 of low reactance being connected betweenthe -grounded cathode of discharge device I and the lower terminal ofresistance 25 removed from the main anode 20. Also, potential variationson electrodes 2| and 22, due to an alternating voltage applied betweenmain control grid I5 and cathode of discharge device I 0 are impressedon the second control grid I8 through a coupling capacitance 29connected therebetween.

Therefore, alternating voltages applied directly to control grid I5 andindirectly to control grid I8 cause alternating output vsignals toappear across resistance 25, which amplified signals are lthen appliedto the grid circuit of another linearly amplifiying, discharge deviceI4. Coupling capacitance 3| and input resistance 32 are connected inseries and the series circuit formed thereby is connected in parallelcircuit relationship to the series circuit formed by output couplingresistance and low reactance by-pass capacitance 28. Capacitance 3| isof relatively low reactance and serves essentially as a means forblocking the flow of continuous current from source 23 to resistance 32.The alternating voltage developed across resistance 32 through condenser3| is applied between the grid and cathode of discharge device I4 so asto control the space current therein, which current normally flows dueto the fact that voltage source 23 is connected between the plate andcathode of discharge device I4 through the primary winding 34A of anoutputJ transformer 35.

Discharge device I4 is preferably of the pentode type having itssuppressor grid connected to the cathode and with the voltage source 23connected between its screen grid and cathode through a voltage droppingresistance 36. The

screen grid is -maintained at constant potential in the presence ofsignals of frequency corresponding to audio-frequencies by means of lowreactance by-pass capacitance 31, which is connected between the screengrid and grounded cathode of device I4.

Alternating voltages developed across resistance 32 are amplifiedlinearly by discharge. device I4 and appearacross the secondary winding39 of transformer 35 which is connected to impress those amplifiedvoltages on the transducer I2 which is connected across the terminals ofa secondary winding 39.

'Thelamentary'cathodes of discharge devices I0 and I4 are .preferablyheated by current flowing therethrough as shown in Fig. 1. In suchcasethe cathodes of discharge devices |||l and I4 are connected in parallelcircuit relationship to voltage source 40.

The circuit thus far described is especially useful as a hearing aidcircuit and, because of the high gain obtained by the use of dischargedevice I0, only two discharge devices requiring duced by transducer I2may be controlled also by connecting high pass filter I1 and capacitancein the hearing aid circuits thus far described.

In general, high pass nlter I'I, which comprises a resistance whentransducer II is a piezoelectric crystal, may be connected in parallelcircuit relationship to transducer II, to reduce the intensity ofthe'low notes. Capacitance 30, which may be connected between theelectrode 2| and cathode of discharge device, tends to reduce theintensity of high notes. Different types of tones may be reproducedcorresponding tothe four positions of the tone control member I3 shownin Figs. 2-4. That is, tone control member I3 is a, short-circuitingmember of such shape that in its clockwise movement it assumes positionswhereby: (1) AS in Fig. 1, the capacitance 30 alone is connected in thehearing aid circuit and high notes only are suppressed, and (2) as inFig. 2, when both lter 'I1 and capacitance 30 are connected in thehearing aid circuit, both some of the high and some of thelow notes aresuppressed, and (3) as in Fig. 3, when the lter I'Iv alone is connectedin the xhearing aid circuit only some of the low notes are suppressed,and (4) as in Fig. 4, when neither the lter I1 nor ,Y the capacitance 30is connected in the hearing aid circuit neither the low notes nor thehigh notes are aiected. It is noted that lead 52 is connected directlyto grid I5 when transducer II comprises a piezoelectric crystal.

As mentioned previously, discharge device I0 operates as vtwo amplifyingstages combined in the envelope of one discharge device withregeneration between the two amplifying stages. The rst stage may beconsidered to comprise a triode section including the cathode ofdischarge device I0, main control grid I5 and electrode 2|, whichelectrode operates through the anode of conventional triode. The secondamplifying stage operates like a pentode and includes control electrodeI8, screen electrode 22, suppressor electrode I9 and anode 20.

Fig. 5 shows in generalized form a structure embodying features of thepresent invention, for purposes of controlling frequency response ortone. This structure, an electro-acoustical device, for' convertingsound energy into electrical energy as in a, microphone or fo'rconverting electrical energy into sound energy as in a speaker,comprises an electro-mechanical or mechanical-'electro transducer 45'mechanically coupled to an acoustical-mechanical ormechanical-acoustical device 46 through a coupling 41,

the transducer 45 being supported in casing 48 by transducer 46 andcasing. 48 has dimensions smaller than a quarter of a wave length x/4)small space current are nec'essary for a good of sound energy in the'audible range. A variable -opening 44 is provided through casing 48 forplacing the space exterior of casing 48 in communication withthe space43 for aiecting the frequency response of one or more components of thecomposite transducer.

Y It is understood that Fig. 5 is representative in showing elements ofwell known electroacoustical transducers controlled in a novel manner.Such transducers may be of the magnetic,

condenser, piezoelectric or carbon types', as is well understood in theart.

The transducers Il, I2 are illustrated herein as being of the magnetictype', it being well known that a transducer of the type shown in Figs.6 and 7 may be used interchangeably as a microphone or as a soundreproducer or speaker.

transducer shown in Figs. 5-7 is used as a sound reproducer receiver orspeaker in the circuit of Fig. 1, the transducer terminals 50 and 5| areI connected, respectively, to lead 54 and ground (Fig. 1) 10 Thetransducer 55 shown in Figs. 6 and 7, which may be used either as amicrophone (Fig. 7) or speaker (Fig. 6) for achievingthe purposes of thepresent invention, is of the type shown and claimed in the copendingapplication, Serial l5 No. 484,153, led April 23, 1943, now Patent No.

2,371,819, of Gilbert E. Gustafson, and assigned to the-same assignee asthe present invention, but modiiied in accordance with the teachings ofthe present invention.

When used as a sound reproducer in a hearing aid circuit, as shown inFigs. 1 and 6, the transducer 55 has a stud- 56 and a resilient earpiece51 assembled `thereon in thev manner described and claimed in thecopending application, Serial No. 507,438, iiled October 23, 1943, ofElizabeth Kelsey, and assigned to the same assignee as the presentinvention. However, the stud 56' and earpiece 51 may be replaced by amolded earpiece.

Also, coupling means 33 (Fig. 1) comprises an 30 impedance matchingtransformer having one of its primary and secondary terminals groundedand the other terminals connected respectively 't0lead52andgrid|5.

When the transducer is used as a micro- 35 phone in a hearing aidcircuit, as shown in Figs.

1 and 7, sound impinging on movable pressure' responsive diaphragm orvibratory element causes a change in current flow through leads enlargedopening in cover 10,"and has its other en d supported in casing 62, thecover 10 beingl of resilient material and being so shaped as to .be heldin adjusted position on casing 62, due to contact pressure and thefrictional forces existing therebetween. It is thus seen that means areprovided for increasing, diminishing or` closing` oi entirely thecross-sectional area of the com-r munication or passageway extendingbetween space and the outside space surrounding casing 62. And it isreadily seen that the series of openings shown in Figs. 6 and 8 maycomprise one large opening, a slotted opening 68 as shown in Fig. ,9, orsuch openings may assume other sizes and shapes.

The purpose of the movable cover 10 and the x variable size openings 61comprising a passageway extending through the casing 62 from space 65into the space surrounding casing 62 is to allow a change in mechanicalresonant frequency ofthe vibratory element or diaphragm 60. That is,

in a practical embodiment of the present inve n tion, it has been foundthat thel mechanical resonant frequency of vibratory element ordiaphragm 60 is increased when the -open cross- -sectional area of thepassageway comprising channels 51 is decreased, and that the resonantfrequency of such vibratory element or diaphragm 50 is decreased 'whenthe open cross-sectional area of the passageway is increased. It hasbeen found also that the length "of port 61 influences l thefrequencyresponse characteristic of thevibratory element.

I'he mechanical' resonant frequency of. the vibratory element 50 isdeterminedl bythe compliance and inertia of such element, and isadjusted in accordance with the teachings of this inveni tion preferablyby varying the compliance of such vibratory element by varying thecross- 66 and 5| from voltage source connected thereto 40 sectional areaof the passageway extending from and also causes a change in current nowthrough resistance I6.

' One of the important features of thepresent invention is that when thetransducer shown in Figs. 6 and 7 is used either as a sound reproducer45 Y said that increasing the'cross-sectional area of ducer but also toimpart a controllable over-all 60 frequency response characteristic tothe composite hearing aid apparatus of which such transducer ortransducers form a part.

The vibratory element or diaphragm 60, of circular shape. is housedwithin casing 62 and de- 55 snes with the inner faces s: nd el an airspace 66 of relatively small volume detlned by linear dimensions lessthan one-quarter of the wave 'length of audible sound signals. This airspa 66, contiguous to the diaphragm 66, is enclosed so by casing 62 anddiaphragm 60, but may be placed in communication 'with the space outsideof casing 62 by means of one or more of a series of portschannels oropenings 61 of the same size or if diifering sizes and extending fromspace 65 66 to the exterior of casingv 62 through such easing. Thecross-sectional area'of the communication or passageway formed bychannels. openings or ports 61 may be selectively controlledby'positioning a. pivoted yplate I6 with respect 7 tosuchopenings.

The cover or channel closure member 16 is movably and adjustablysupported on casing 62 by providing-ra pivot pin-12 which has an enspace65 to the exterior portion of casing 62. It has been observed, also,that the amplitude of vibration of vibratory element 50 increases withan increase in the cross-sectional areal of the passageway comprisingports 61. It can be thus the passageway not onlyl causes-a. change indamping-of or resistance to movement of vibra- .tory element, but alsothat a. mechanical imreactance elements, as distinguishedfrom'distributed reactance elements.- with no undesirable resonant andanti-resonant frequencies in the transmitted range, such lumpedreactance elements being of a nature to enectively change the I resonantfrequency of the vibratory element.

mechanical resonant frequency lof the vbratingelements in thetransducers I l and4 I2 may thus be varied by varying the position ofplate 16 on casing 62. Fig. 10 shows two -fre-A quency responsecharacteristics 86 and 6l of),

lar-ged portion on one end, passes through an transducer 'havingcommunications of dido-at cross-sectional area extending from space 65to the space exterior of casing 62. Characteristick 80 is obtained whenthe area of the communication is relatively large and characteristic 8|is obtained when the area, lof such communication is relatively small.It is noted that the maximum amplitude of characteristic 80 greater thanthe maximum amplitude of characteristic 8l. This dierence is perhaps dueto increased damping of or resistance to movement.

of diaphragm 60 when the area of the communication is relatively small.

It is further seen that means are provided for shifting the frequency atwhich the amplitude response of a transducer occurs. This shifting ofthe frequency at which the maximum amplitude occurs is, of course,reflected in the shape of the over-all frequency response characteristicof the composite hearing aid or other audio circuit. In fact, suchover-all frequency response characteristie may be controlled by shiftingthe frequency at which such maximum amplitude of the characteristic ofone or both of the transducers II and I2 occurs. By this means, personshaving different types of hearing deficiency and using hearing aidequipment incorporating such controllable transducers mayv readilyadjust the cover 10 so as to select that over-all frequency responsecharacteristic which they consider best for hearing. l

Although the position of cover 'I0 determines the frequency at which themaximum amplitude on the frequency response characteristic of theassociated transducer occurs and also aiects the over-al1 frequencyresponse characteristic of the associated apparatus, the position oftone control is somewhat t I3 produces an additional effect, of afiltering Y type, on the over-all frequency response characteristic ofsuch apparatus.

That is, it can be said categorically that the position of cover IIIdetermines the position of maximum peaks on the over-all frequencyresponse characteristic of the apparatus shown in Fig. 1 and that theposition of tone control member I3 affects the shape of the over-allfrequency response characteristic by affecting the degree towhich'signals are amplified over a band of frequencies.

It is understood that other types of tone conl transducers I I and vI2where such transducers are of the impedance or piezoelec ric type. Whensuch electrical tuners 85 and/or 86 are used for effecting tone control,corresponding switches 81 and/or 88 are closed and the correspondingtransducers II and/or I2 may be tuned electrically.

When the transducers II and l2 are of the condenser or piezoelectrictype, it is readily apparent to one skilled in the art that tuners 85and 86 should be variable inductances for tuning the transducer circuitand that when the transducers II and I2 are of the magnetic type suchtuners should be variable condensers.

It is readily seen that means are provided not only for tuning thetransducers II and I2 mechanically to a desired frequency, but alsomeans vare provided for taining such transducers II and I2 electricallyto the same, or preferably to another, frequency by varying tuners and86. It.

is also apparent'that the over-all frequency rresponse characteristic ofthe hearing aid circuit shown in Fig. 1 may be controlled separately orsimultaneously in five different ways, namely, (1) by mechanicallytuning transducer II, (2) by mechanically tuning transducer I2, (3) byadjusting tone control element I3, (4) by 'electrically tuningtransducer I l, and (5) by electrically tuning transducer I2. It isfurther apparent that, with the means shown herein, persons havinghearing deficiencies of variously different types are provided withmeans or a combination of means allowing them to adjust the over-allfrequency response characteristic of hearing aid apparatus for suitableamplified understandable hearing.

While the particular embodiments of the pres- .ent invention have beenshown and described, it

will be obvious to those skilled in the art that.

changes and modifications may lbe made Without departing from thisinvention in its broader aspects, and, therefore, the aim in theappended claims is to cover all such changes and modications as fallwithin the true spirit and `cope of this invention.

I claim:

1. In combination, an electro-acoustical trans'- ducer having dimensionssuitably small for inconspicuous wear and a wall. said transducer havinga movable diaphragm with a face thereof enclosed by the wall, and meansfor altering a physical parameter of said transducer in such an amountthat the mechanical frequency response is substantially affected, saidmeans including a sound channel extending through said wall in such a,direction that movement of the air produced by movement of said facetoward said wall is away from the diaphragm.

` 2. In a hearing aid apparatus operable in a band of frequencies, aninput transducer Yand an output transducer, each of said transducershaving a vibratory member with an inherent'mechanical resonant frequencyin said band of frequencies whose effect is reflected in selectivelyreproducing vibratory energy of a frequency within said band offrequencies, and means for altering the mechanical resonant frequenciesof each of said vibratory members whereby the shape of the over-allfrequency response characteristic of said apparatus may be controlled.

3. In electrical apparatus operable over a range of audible frequenciesand having a variable frequency response characteristic, a vibratorymemlber, a housing for said vibratory member enclosing a, face ofthevibratory member, said housing enclosing a space determined by lineardimensions smaller than one-quarter of the wave length of signals insaid audible'range, and means for altering a characteristic of saidhousing whereby the frequency response characteristic of .said apparatusis controlled, said means including' an air channel extending throughsaid housing in such a direction that movement of the a, 1 produced bymovement of said face toward said housing is away from the vibratorymember.

4. In signal translating apparatus having an over-all frequency responsecharacteristic and including an input circuit, an electro-acousticaltransducer in said input circuit, said transducer including a vibratorymember having a mechanical characteristic which causes said transducerto have a. peaked frequency characteristic Within the range offrequencies translated by said apparatus,

- a casing enclosing a face of said vibratory member, and mechanicalmeans for controllingthe position of said peaked frequencycharacteristic in said frequency range, said means including a soundopening extending through said casing in f such a direction thatmovement of the air produced by movement of said face toward said casingis away from the vibratory member, whereby the over-all frequencycharacteristic of said apparatus is aected.

5. In signal translating apparatus having an over-al1l frequencyresponse characteristic and including an energy output circuit, anelectroacoustcal transducer in said output circuit, said transducerincluding a vibratory member having a mechanicalcharacteristic whichcauses said transducer to have a peaked frequency characteristic withinthe range of frequencies translated by said apparatus, a casingenclosing a face of said vibratory member, and mechanical means forcontrolling the position of said peaked frequency characteristic in saidfrequency range, said means including a sound opening extending throughsaid casing in such a direction that movement of the air produced bymovement of said face toward saidl casing is away from the vibratorymember, whereby the over-.al1 frequency characteristic of said apparatusis affected.

6.,In signal translating apparatus having an over-al1 frequency rponsecharacteristic and -including an input circuit and an energy outputwhich causes said transducer to have a peaked frequency characteristicwithin the range of frequencies translated by said apparatus, a casingenclosing a face of said vibratory member, me-4 lating device operableover a band of audible frequencies and having an input and an outputcircuit, a first electro-acoustical transducer oonnected t0 said inputcircuit, a second electroacoustical transducer connected to said outputterminalaat least one of said transducers having a vibratile elementtunable to a plurality of mechanical resonant frequencies within saidrange of frequencies, a housing enclosing a face of said -vibratileelement the mechanical resonant frequencies of said ,vibratile elementsproducing a substantial effect on the frequency response characteristicof said apparatus, means for mechanically tuning said vibratile memberfor controllingi said characteristic, said means including an airchannel extending through said wall in such a direction that movement.of air produced by movement of said face toward said wall is away fromthe vibratile element and additional means in said translating devicefor controlling said characteristic.

9. In combination, electrical apparatus including an electro-acousticaltransducer and having a controllable over-all frequency responsecharacteristic, said transducer having a vibratile element whoseresonant frequency aiects sulbstanf tially said over-all frequencyresponse characteristic of said apparatus, a housing enclosing a facechanical means for controlling the position of said peaked frequencycharacteristic in said frequency range, said means including a soundopening extending through said casing in such adirection that movementof the air produced by movement of said face toward said casing is awayfrom the `vibratory member, whereby the over-all frequencycharacteristic of said apparatus is affected, a secondelectro-acoustical transducer insaid output circuit, said secondtransducer including a second vibratory member having a mechanicalcharacteristic which causes said second transducer to have a peakedfrequency characteristic within the range of frequency translated bysaid apparatus, a second casing enclomng la face of said secondvibratory member and mechanical means for controlling the position ofsaid last mentioned peaked frequency characteristic in said frequencyrange. said means includof said vibratile element, and means forvariably and mechanically turing said vibratile element, said meansincluding an air channel extending through said wall in such a directionthat movement of air producedby movement of said face toward said wallis away from the vibratile element, whereby a selective control eifectmay be exerted on said characteristic..

10. In an electro-acoustical transducer having a variable frequencyresponse characteristic, a. member movable in response to energyimparted40 thereto of a wave length within an audible'frequency range, a housingenclosing a face of said ing a sound opening extending throughsaid'second casing in such a direction that movement of airl produced bymovement of said face toward.

said second casing is away from the vibratory member, wherebytheover-all frequency characteristic of said amiaratus fs additionallyaffected.

7. In electro-acoustic apparatus operable over a range of frequencies. atransducer having an electrical impedance and a vibratory element havingan adjustable mechanical resonant frequency, means for tuning saidtransducer electrically over at least a portion of said range offrequencies. and means for tuning said vibratory element mechanicallyover 'at least a portion of said range of frequencies whereby thefrequency response characteristic of said apparatus is con'- trolled.

8. In electro-acoustical. apparatus having a desired frequency responsecharacteristic over ,a range of frequencies, an audio-frequency transthefrequency responseV characteristic of said transducer is controlled.

11. In an electro-acoustical transducer having variable `frequencyresponse characteristics, a

member movable in response to vibratory energy of frequencies within anaudible range, said member having a dennite pronounced peaked amplitudevariation as a function of applied vibratory energy applied theretowithin said audible range,

a closure member for one face of said movable member, said closuremember enclosing a volume defined by linear dimensions smaller thanonequarter of the wave length of saidl vibratory eri-g ergy within Vsaidaudible range, and a variable opening extending through said closuremember in such a direction that movement of air produced by movement ofsaid face toward said closure member is away from the movable memberwhereby thefrequency response of said transopenin tween said input andoutput transducers, each of said transducers having a, vibratory memberwith an inherent mechanical resonant frequency in said band offrequencies whose effect is reflected in selectively reproducingvibratory energy of a frequency within said band of frequencies, meansfor altering the mechanical resonant frequencies of each of saidvibratory members whereby the shape of the over-all frequency responsecharacteristic of said apparatus may be controlled, and additional meansin said translating means for controlling said characteristic.

13. In electrical apparatus operable in a band of frequencies, an inputtransducer and an output transducer, each of said transducers having avibratory member with an inherent mechanical resonant frequency in saidband of frequencies Whose effect is reflected in selectively reproducingvibratory energy of a frequency within said band of frequencies, meansfor altering the mechanical resonant frequencies of each of saidvibratory members whereby the shape of the over-all frequency responsecharacteristic may be controlled, and means for tuning at least one ofsaid transducers electrically over at least a portion of saidY range offrequencies to additionally control said characteristic.

14. In electro-acoustic apparatus operable over a range of frequencies,a transducer having an electrical impedance and a vibratory elementhaving an adjustable mechanical resonant frequency, said apparatusincluding a translating means connected to said transducer, means fortuning said transducer electrically over at least a portion of saidrange of frequencies, means for tuning said .vibratory elementmechanically over at least a portion of said range of frequencies,whereby the frequency response characteristic of said apparatus iscontrolled, and additional means in said translating means forcontrolling said characteristic.

15. The invention defined by claim 8 characterized by means for tuningelectrically at least one of said transducers to a frequency within saidrange of frequencies.

the frequency response characteristic of said ap-v paratus iscontrolled.

17. 1n combination, an electro-acoustical transducer having dimensionssuitably small for inconspicuous wear and a housing, a movable diaphragmarranged within said housing to divide the space within said housinginto two parts, one part of said space being substantially entirelyenclosed bysaid diaphragm and said housing, said housing being arrangedto form a communicationl between the other part of said space and theouter ear of a person wearing said transducer in which case said otherpart of said "nace is entirely enclosed by said diaphragm, housing andouterear of the user and out of air communica-- tion with said one partof said space, and means for altering a physical parameter of saidtransducer in such an amount that the mechanical frequency response issubstantially effected, said means including a sound ,channel extendingthrough said housing to place said one space in communication with theatmosphere surrounding said housing whereby when said transducer is fworn by a user no air communication exists between the two sides of saiddiaphragm and the mechanical frequency response characteristic is vsubstantially effected by the presence of said sound channel extendingthrough said housing to place only said one part of said `.pace incommunication with the atmosphere.

COSLER. DONALD MIGHT.

